[fix] be able to silence more warnings

[dummdidumm]

including css-unused-selector, unused-export-let, module-script-reactive-declaration
Fixes #5954
Related to #5281

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[benmccann]

FYI, @dummdidumm it looks like this PR will need to be rebased